The invention is in the field of fluorescent lamp systems having one or more fluorescent lamps connected in electrical series with a ballast, for operation in the 200 to 300 volt range, such as standard values of 220, 240, or 277 volts.
Many fluorescent lamp systems have two lamps, usually positioned side-by-side, to provide more light from a larger source area than does a single lamp. Fluorescent lamps are manufactured in various different lengths, the 24-inch length and the 48-inch length being widely used in lighting systems. The voltage required for starting and operating fluorescent lamps increases approximately linearly with increased lamp length. Thus, a 48" lamp requires twice the voltage as a 24" lamp, two 24" lamps connected in series require essentially the same voltage as a single 48" lamp, and two 48" lamps in series require approximately twice the voltage as two 24" lamps in series.
A widely used type of fluorescent lamp circuit utilizes a voltage step-up transformer connected between the lamp(s) and the power line voltage supply, for providing suitable voltage to the lamp(s), such as is disclosed in U.S. Pat. No. 4,185,233 to Riesland et al. Another widely used fluorescent lamp circuit, which is more economical to manufacture, utilizes a current-limiting ballast impedance (such as an inductor, resistor, and/or capacitor) connected in series between the lamp(s) and the power line voltage source. The total length of the discharge path(s) of the lamp(s) in this circuit must be short enough so the lamp starting and operating voltages are sufficiently less than the line voltage. Therefore, such circuits have been limited to operating a single 24" (or shorter) fluorescent lamp from a 120-volt a-c line, and two 24" (or shorter) lamps connected in series (or a single 48" or shorter lamp) from a 220-volt, 240-volt, or 277-volt a-c line, the latter voltage being obtained from a 480-volt three-phase system. For convenience, these voltages are defined herein as being in a range of 200 to 300 volts.
FIG. 1 of the drawing shows a prior art two-lamp series-ballast circuit for operation from a standard line voltage in the range of 200 to 300 volts a.c. at a standard line frequency of 50 Hz or 60 Hz. A pair of conventional fluorescent lamps 11, 12, each of the 24-inch (20-watt) type, each contains a pair of cathodes 11a, 11b and 12a, 12b near the ends of the elongated 24-inch bulbs 11', 12'. An end of cathode 11b is connected to an end of cathode 12b by a conductor 13, thus connecting the lamps 11, 12 in electrical series. An end of cathode 12a is connected to a line voltage terminal 14, and a ballast inductor 16 and capacitor 17 are connected in series between an end of cathode 11a and the other line voltage terminal 18. The line voltage at terminals 14 and 18 is in the range of about 200 to 300 volts, e.g., a standard a-c line voltage of 220 volts, plus or minus ten percent, 240 volts, plus or minus ten percent, or 277 volts, plus or minus ten percent. The inductor 16 ballasts the lamps, i.e., it limits their operating current to a desired and safe value, and the capacitor 17 functions to shift the circuit power factor from lagging to leading, and also resonates with the inductor to increase the starting voltage for the lamps, in well-known manner.
A starter switch 21 is connected across the remaining ends of cathodes 11a and 11b, and a starter switch 22 is connected across the remaining ends of cathodes 12a and 12b. The starter switches 21, 22 may be the well-known glow starter switch, and they function by closing when the electrical power is first applied to the circuit, thereby causing preheat current to flow through the cathodes to heat them sufficiently to readily emit electrons to sustain a discharge current in the lamps. After about a second, the starter switches open and electric discharges occur in the lamps, causing a phosphor coating on the bulb to emit visible light.
It has been desirable to be able to have a lighting system employing a pair of 48-inch fluorescent lamps in series with a simple series ballast as shown in FIG. 1 for operation from a line voltage in the range of 200 to 300 volts, but this has not been achieved heretofore because of the higher voltage requirements of the 48-inch lamps as compared to the 24-inch lamps. Such a system, if it could be achieved, would provide about twice the light output of a 24-inch lamp system with a substantial increase in system efficacy at only a small increase in cost.